In this paper off-state drain currents in long-channel inversion mode SOI MOSFETs are investigated in the range 50-320oC by measurements and simulations. The behavior of high-temperature (T>200oC) off-state currents is interpreted in terms of diffusion model, based on the analysis of carrier distribution in a SOI film in off-state of the device. The back-gate biasing and the silicon film thinning effects on high-temperature off-state currents are analyzed.